This invention relates to an apparatus for detecting a mutual positional relationship of two members using a pair of diffraction gratings.
A technique for highly accurately aligning two objects, for example a semiconductor wafer and mask, with each other using a pair of diffraction gratings is disclosed in "A new interferometric alignment technique" Applied Physics Letter, vol. 31, No. 7 (1977) p. 426 and "Alignment of X-ray lithography masks using a new interferometric technique-Experimental results" J. Vac. Sci. Technol. 15(3) May/June (1978) p. 984, both being written by S. Austin, D. C. Flanders, H. I. Smith et al. In the first reference entitled to "A new interferometric alignment" first and second diffraction gratings 1 and 2 having the same pitch are arranged in parallel with each other as schematically shown in FIG. 1 herein and a coherent light is vertically directed by a light source 3 at the side of the second diffraction grating 2 toward the diffraction gratings. Those +n- and -n-order beams diffracted on the diffraction gratings are received by light receiving devices 4, 5 and the intensities of the received beams are both displayed on an indicator 6. If there is a displacement .DELTA.x between the pitches of the diffraction gratings the light beams diffracted on the diffraction gratings 1, 2 are dephased with respect to each other with the result that the +n- and -n-diffracted beams suffer mutually different diffraction modes. For this reason, the light beams incident onto both the light receiving devices 4, 5 are different in their amount and are so displayed on the indicator 6. If in this case the first and second diffraction gratings 1 and 2 are moved in the horizontal direction relative to each other until the amounts of light beams incident onto the light receiving devices 4 and 5, respectively, are made to coincide with each other, the above-mentioned displacement .DELTA.x ceases to exist, thus completing a positional alignment.
In the second reference entitled "Alignment of X-ray lithography masks using a new interferometric technique-Experimental results", a coherent light is directed by a light source at the side of a first diffraction grating 1 toward the first and second diffraction gratings 1 and 2 as schematically shown in FIG. 2 herein. The light beams diffracted on the diffraction gratings are received by detectors 4, 5 and a positional alignment is effected as in the case of the first reference.
The above-mentioned positional alignment techniques have an advantage of being capable of effecting a positional alignment with a high accuracy determined by the pitch of the diffraction gratings 1, 2 and wavelength .lambda. of the coherent light. When, however, the positional alignment is to be effected using these techniques, if a distance D between the diffraction gratings varies during the horizontal movement of the gratings relative to each other, the optical length varies. As a result, the phase of the diffracted beams greatly varies, thus making it very difficult to judge a positional displacement. If such technique is actually applied to various apparatus it would be almost impossible to prevent a variation in the distance D between the gratings from occurring due to, for example, floor vibrations and vibrations of movable operating members. Furthermore, even if other beams (disturbed beams) than the diffracted beams are incident onto the devices 4, 5 it would be impossible to make accurate detection and, therefore, greatest care has to be exercised against such disturbed beams.